1. Field of the Invention
This invention relates to multi-mode interference (MMI) optical waveguide devices.
2. Description of Related Art
U.S. Pat. No. 5,410,625 describes a multi-mode interference (MMI) device for beam splitting and recombining. The device comprises a first coupling waveguide and two or more second coupling waveguides that are connected to a central multi-mode waveguide region. The coupling waveguides operate only in fundamental mode, and the physical characteristics of the coupling and multi-mode waveguide regions are selected such that modal dispersion within the central multi-mode waveguide region provides for a single beam of light input in to the first coupling waveguide to be split into the two or more second coupling waveguides. The device may also be operated in reverse as a beam combiner.
Variations and improvements to the basic MMI devices of U.S. Pat. No. 5,410,625 are also known. U.S. Pat. No. 5,379,354 describes how variation of input guide location can be used to obtain a multi-way beam splitter that provides division of the input radiation into outputs beams having differing intensities. Use of MMI devices to form laser cavities has also been demonstrated in U.S. Pat. No. 5,675,603. Various combinations of MMI splitter and recombiner devices have also been used to provide an optical routing capability; for example, see U.S. Pat. No. 5,428,698.
Solid core MMI waveguide devices are known in which the coupling and multi-mode waveguides are formed from solid ridges of semiconductor material, such as Gallium Arsenide (GaAs), that are upstanding from a substrate. Solid core MMI waveguide devices are typically fabricated from layers of GaAs. A disadvantage of solid core materials is the limited total power density which they can transmit before damage to the solid core material occurs.
MMI devices are also known in which the coupling and multi-mode waveguides are formed as hollow cavities (i.e. air cavities) within substrates of solid dielectric material, such as alumina. The dielectric substrate material is selected to have a refractive index less than the air core at the particular wavelength of operation of the device. Hollow core dielectric devices are typically fabricated by a precision engineering (e.g. milling) process and are typically physically larger in size than their solid core counterparts. Precise control over the dimensions of such devices, which is important for obtaining optimum performance, can also prove challenging.